1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to a band filter using a film bulk acoustic resonator and a method of fabricating the same, and more particularly to a band filter using a film bulk acoustic resonator in which a structure of a cavity for vibrating a thin film is modified and a method of fabricating the same.
2. Description of the Related Art
Generally, a film bulk acoustic resonator (FBAR) is a filter using a bulk acoustic wave of a piezoelectric layer. A size of a general frequency filter is proportional to a wavelength of an electromagnetic wave in a usage frequency band. Therefore, the size of a general frequency filter using the electromagnetic wave is relatively large. For example, when the frequency of the electromagnetic wave is 1 GHz, the size of a general frequency filter is approximately 30 cm, and when the frequency of the electromagnetic wave is 300 GHZ, the size of a general frequency filter is approximately 1 mm. However, if the bulk acoustic wave of, the piezoelectric layer is used, a wavelength of the bulk acoustic wave becomes less as a fraction ( 1/10,000) of a wavelength of the electromagnetic wave. According to this, the electromagnetic wave is converted into the bulk acoustic wave by the piezoelectric layer, and the size of the filter becomes less in proportion to the wavelength of the bulk acoustic wave. That is, since the size of the frequency filter using the bulk acoustic wave is approximately several hundred microns, and a plurality of the frequency filters using the bulk acoustic wave can be fabricated at one time by using a wafer, mass production of a band filter is possible.
FIG. 1 is a schematic plane view showing a duplexer filter with a film bulk acoustic resonator in accordance with a related art.
Referring to FIG. 1, a duplexer filter 1 includes a substrate 3, a transmission-side film bulk acoustic filter 5 and a reception-side film bulk acoustic filter 7, formed on the substrate 3.
The transmission-side film bulk acoustic filter 5 and the reception-side film bulk acoustic filter 7 are constituted by film bulk acoustic resonators 10 formed on the substrate 3 and connected in series or parallel.
FIG. 2 is a cross-sectional view showing a filter with cavities of different sizes, taken along line I-I′ shown in FIG. 1.
Referring to FIG. 2, a plurality of cavities 3a, 3b and 3c with different sizes are formed on a substrate 3 and a membrane layer 4 is applied over the substrate 3 to cover the cavities 3a, 3b and 3c. Resonators 10 are formed on the membrane layer 4 at positions corresponding to the cavities 3a, 3b and 3c. A first electrode 11, a piezoelectric layer 13 and a second electrode 15 stacked sequentially constitutes a resonator 10.
A method of manufacturing the filter described above is explained briefly below.
FIG. 3A to FIG. 3C illustrate a method of fabricating a duplexer filter with film bulk acoustic resonators and FIGS. 4A and 4B are views for explaining the loading effect and the notches generated upon dry etching in accordance with the related art.
Referring to FIG. 3A, a membrane layer 4 is deposited on a substrate 3 and resonators 10 are created on the membrane layer 4.
Referring to FIG. 3B, a mask layer 9 is formed on the opposite surface of the substrate 3 with respect to the membrane layer 4 and patterned to produce windows 9a, 9b and 9c at positions where cavities 3a, 3b and 3c would be formed.
Referring to FIG. 3C, the substrate 3 is etched through the widows 9a, 9b and 9c using an inductively coupled plasma (ICP) etching equipment so that the cavities 3a, 3b and 3c are formed in the substrate 3, leaving walls 3d every between of the cavities 3a, 3b and 3c. 
However, in accordance with the related art described above, in case of simultaneously etching the substrate 3 to form the cavities 3a, 3b and 3c with different sizes using the ICP etching equipment, it is difficult to precisely produce the cavities 3a, 3b and 3c with desired sizes due to a loading effect and a lag effect.
The loading effect means that edge portions of the cavities 3a, 3b and 3c, denoted by alphabetical reference “L” in FIG. 4A remain not being etched due to relatively low etch rate at the edge portions of the cavities 3a, 3b and 3c compared to the etch rate of the center portions of the cavities 3a, 3b and 3c. 
The lag effect is generated because etch rates for every cavity 3a, 3b and 3c are different depending on the sizes of the cavities 3a, 3b and 3c. That is, etch rates in larger cavities are relatively higher than that in the small cavities. Accordingly, even if the etching is simultaneously performed under the same condition for every cavity 3a, 3b and 3c, the cavity 3c will be larger than the desired size after an etching process in a case where the size of the cavity 3c is larger than that of the others.
The notch effect means that the substrate 3 is over-etched at the bottom of the cavities, i.e. near the membrane layer 4, thereby forming notches in the walls 3d. The notch denoted by alphabetical reference “N” in FIG. 4B is created since ions generated during the etching process are bombarded to and reflected from the membrane layer 4 and the reflected ions etch the walls 3d by bombarding the walls 3d. 
As described above, in a case where all the cavities 3a, 3b and 3c with different sizes are etched at the same time, it is difficult to form the cavities 3a, 3b and 3c with desired sizes due to the loading, lag and notch effects. Further, even the membrane layer 4 can be damaged due to a stress and walls 3d could be eliminated.